[PATCH] mm/slub: clamp slab_min_order to MAX_PAGE_ORDER

[PATCH] mm/slub: clamp slab_min_order to MAX_PAGE_ORDER

[Sang-Heon Jeon]

setup_slub_max_order() clamps slab_max_order to MAX_PAGE_ORDER, but
setup_slub_min_order() has no such clamp.

The buddy allocator cannot allocate more than 2^MAX_PAGE_ORDER pages
at once, so slab_min_order above it makes no sense.

If slab_min_order is set above MAX_PAGE_ORDER, it can even panic at
boot or invoke undefined behavior in order_objects().

So clamp slab_min_order to MAX_PAGE_ORDER as well.

Fixes: 818cf5909701 ("slub: enforce MAX_ORDER")
Cc: stable@vger.kernel.org
Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
---
QEMU-based test results

- Booted with "slab_min_order=20"

1) AS-IS (before fix)
[    0.094926] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
[    0.097776] Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22
[    0.099542] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.2.0 #1 PREEMPT(undef) 
[    0.100903] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[    0.102607] Call Trace:
[    0.103040]  <TASK>
[    0.103417]  vpanic+0x32a/0x4b0
[    0.103976]  panic+0x5e/0x60
[    0.104491]  create_boot_cache+0xe3/0xf0
[    0.105194]  kmem_cache_init+0x12d/0x410
[    0.105890]  ? mem_init+0x1fc/0x210
[    0.106521]  mm_core_init+0xba/0x140
[    0.107153]  start_kernel+0x441/0x8b0
[    0.107814]  x86_64_start_reservations+0x18/0x30
[    0.108638]  x86_64_start_kernel+0x10c/0x120
[    0.109401]  common_startup_64+0x13e/0x158
[    0.110145]  </TASK>
[    0.110584] ---[ end Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22 ]---

2) TO-BE (after fix)
[    0.008976] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
[    0.011366] SLUB: HWalign=64, Order=10-10, MinObjects=0, CPUs=4, Nodes=1

Also, I'm not sure about the Fixes commit. Please change it if you know
a better one.

---
 mm/slub.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index 1559cbace469..8400df7c222e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -8354,6 +8354,8 @@ static int __init setup_slub_min_order(const char *str, const struct kernel_para
 	if (ret)
 		return ret;
 
+	slub_min_order = min_t(unsigned int, slub_min_order, MAX_PAGE_ORDER);
+
 	if (slub_min_order > slub_max_order)
 		slub_max_order = slub_min_order;
 

base-commit: d755441b7672c74f29d1241ffdd2b067ecab1ca0
-- 
2.43.0

Re: [PATCH] mm/slub: clamp slab_min_order to MAX_PAGE_ORDER

[Harry Yoo]

On Thu, Oct 08, 2026 at 01:34:02AM +0900, Sang-Heon Jeon wrote:
> setup_slub_max_order() clamps slab_max_order to MAX_PAGE_ORDER, but
> setup_slub_min_order() has no such clamp.
> 
> The buddy allocator cannot allocate more than 2^MAX_PAGE_ORDER pages
> at once, so slab_min_order above it makes no sense.
> 
> If slab_min_order is set above MAX_PAGE_ORDER, it can even panic at
> boot or invoke undefined behavior in order_objects().
> 
> So clamp slab_min_order to MAX_PAGE_ORDER as well.
> 
> Fixes: 818cf5909701 ("slub: enforce MAX_ORDER")

I think Fixes: should be the commit that introduced slub_min_order.

> Cc: stable@vger.kernel.org

And no need for Cc: stable? It's been there for decades and people
should realize the kernel parameter is broken when it doesn't boot.

Otherwise looks good to me.

> Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> ---
> QEMU-based test results
> 
> - Booted with "slab_min_order=20"
> 
> 1) AS-IS (before fix)
> [    0.094926] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
> [    0.097776] Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22
> [    0.099542] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.2.0 #1 PREEMPT(undef) 
> [    0.100903] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [    0.102607] Call Trace:
> [    0.103040]  <TASK>
> [    0.103417]  vpanic+0x32a/0x4b0
> [    0.103976]  panic+0x5e/0x60
> [    0.104491]  create_boot_cache+0xe3/0xf0
> [    0.105194]  kmem_cache_init+0x12d/0x410
> [    0.105890]  ? mem_init+0x1fc/0x210
> [    0.106521]  mm_core_init+0xba/0x140
> [    0.107153]  start_kernel+0x441/0x8b0
> [    0.107814]  x86_64_start_reservations+0x18/0x30
> [    0.108638]  x86_64_start_kernel+0x10c/0x120
> [    0.109401]  common_startup_64+0x13e/0x158
> [    0.110145]  </TASK>
> [    0.110584] ---[ end Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22 ]---
> 
> 2) TO-BE (after fix)
> [    0.008976] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
> [    0.011366] SLUB: HWalign=64, Order=10-10, MinObjects=0, CPUs=4, Nodes=1
> 
> Also, I'm not sure about the Fixes commit. Please change it if you know
> a better one.
> 
> ---
>  mm/slub.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 1559cbace469..8400df7c222e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -8354,6 +8354,8 @@ static int __init setup_slub_min_order(const char *str, const struct kernel_para
>  	if (ret)
>  		return ret;
>  
> +	slub_min_order = min_t(unsigned int, slub_min_order, MAX_PAGE_ORDER);
> +
>  	if (slub_min_order > slub_max_order)
>  		slub_max_order = slub_min_order;
>  
> 
> base-commit: d755441b7672c74f29d1241ffdd2b067ecab1ca0
> -- 
> 2.43.0
> 

-- 
Cheers,
Harry / Hyeonggon

Re: [PATCH] mm/slub: clamp slab_min_order to MAX_PAGE_ORDER

[Sang-Heon Jeon]

Hi, Harry

2026년 10월 8일 (목) 오전 2:47, Harry Yoo <harry@kernel.org>님이 작성:
>
> On Thu, Oct 08, 2026 at 01:34:02AM +0900, Sang-Heon Jeon wrote:
> > setup_slub_max_order() clamps slab_max_order to MAX_PAGE_ORDER, but
> > setup_slub_min_order() has no such clamp.
> >
> > The buddy allocator cannot allocate more than 2^MAX_PAGE_ORDER pages
> > at once, so slab_min_order above it makes no sense.
> >
> > If slab_min_order is set above MAX_PAGE_ORDER, it can even panic at
> > boot or invoke undefined behavior in order_objects().
> >
> > So clamp slab_min_order to MAX_PAGE_ORDER as well.
> >
> > Fixes: 818cf5909701 ("slub: enforce MAX_ORDER")
>
> I think Fixes: should be the commit that introduced slub_min_order.

Thanks for your guidance. I'll switch this to 81819f0fc828 ("SLUB core") in v2.

> > Cc: stable@vger.kernel.org
>
> And no need for Cc: stable? It's been there for decades and people
> should realize the kernel parameter is broken when it doesn't boot.

Okay, I'll remove it in v2.

> Otherwise looks good to me.

Thanks for the review :)

> > Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> > ---
> > QEMU-based test results
> >
> > - Booted with "slab_min_order=20"
> >
> > 1) AS-IS (before fix)
> > [    0.094926] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
> > [    0.097776] Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22
> > [    0.099542] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 7.2.0 #1 PREEMPT(undef)
> > [    0.100903] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> > [    0.102607] Call Trace:
> > [    0.103040]  <TASK>
> > [    0.103417]  vpanic+0x32a/0x4b0
> > [    0.103976]  panic+0x5e/0x60
> > [    0.104491]  create_boot_cache+0xe3/0xf0
> > [    0.105194]  kmem_cache_init+0x12d/0x410
> > [    0.105890]  ? mem_init+0x1fc/0x210
> > [    0.106521]  mm_core_init+0xba/0x140
> > [    0.107153]  start_kernel+0x441/0x8b0
> > [    0.107814]  x86_64_start_reservations+0x18/0x30
> > [    0.108638]  x86_64_start_kernel+0x10c/0x120
> > [    0.109401]  common_startup_64+0x13e/0x158
> > [    0.110145]  </TASK>
> > [    0.110584] ---[ end Kernel panic - not syncing: Creation of kmalloc slab kmem_cache_node size=64 failed. Reason -22 ]---
> >
> > 2) TO-BE (after fix)
> > [    0.008976] mem auto-init: stack:all(zero), heap alloc:off, heap free:off
> > [    0.011366] SLUB: HWalign=64, Order=10-10, MinObjects=0, CPUs=4, Nodes=1
> >
> > Also, I'm not sure about the Fixes commit. Please change it if you know
> > a better one.
> >
> > ---
> >  mm/slub.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 1559cbace469..8400df7c222e 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -8354,6 +8354,8 @@ static int __init setup_slub_min_order(const char *str, const struct kernel_para
> >       if (ret)
> >               return ret;
> >
> > +     slub_min_order = min_t(unsigned int, slub_min_order, MAX_PAGE_ORDER);
> > +
> >       if (slub_min_order > slub_max_order)
> >               slub_max_order = slub_min_order;
> >
> >
> > base-commit: d755441b7672c74f29d1241ffdd2b067ecab1ca0
> > --
> > 2.43.0
> >
>
> --
> Cheers,
> Harry / Hyeonggon

Best regards,
Sang-Heon Jeon